It has been known pyrrolopyridazinone compounds similar to the compound of the present invention (see Patent literature 1 and Non-patent literatures 1 to 3), but it has been not known that these compounds have a PDE4 inhibitory activity.
As a PDE4 inhibitor, it has already been known compounds such as xanthine derivatives, Rolipram analogues, or nitroquazone derivatives, etc., but these inhibitors could not be used clinically since they causes severe vomit or nausea as side effects (see Non-patent literature 4). In recent years, PDE4 inhibitors improved in side effects such as vomit and nausea have been reported that they are useful for treatment of asthma, chronic obstructive pulmonary disease (hereinafter also referred to as COPD) and allergic coryza (see Non-patent literatures 5 to 7).
However, it cannot be said that they have a clinically sufficient effects even in compounds in which vomit or nausea as side effects are improved with a certain extent.    Patent literature 1: WO 01/96336 pamphlet    Non-patent literature 1: Chem. Pharm. Bull., 50, 754 (2002)    Non-patent literature 2: Heterocycles, 43, 1863 (1996)    Non-patent literature 3: Heterocycles, 60, 2471 (2003)    Non-patent literature 4: Exp. Opin. Ther. Patents, 7, 989 (1997)    Non-patent literature 5: Lancet, 358, 265 (2001)    Non-patent literature 6: J. Allergy Clin. Immunol., 108, 530 (2001)    Non-patent literature 7: Eur. Respir. J., 10, 1008 (1997)